Transmitter circuits, receiver circuits and input/output (I/O) buses are used extensively in integrated circuits (ICs) both for inter-chip and off chip signaling. Transmitter circuits, receiver circuits and I/O buses are designed to send and/or receive binary data signals through a transmission line. These devices can be used, for example, in a computer's system bus, in internal busses and system bus interface units of a CPU, and in memory devices such as DRAM.
Integrated circuit devices having upwards of several hundred I/O lines and associated transmitter, receiver, and or transceiver circuits are common. A transceiver circuit can have a transmitter circuit for sending signals and a receiver circuit for receiving signals. Transmitter and receiver circuits can occupy considerable area on an integrated circuit. Additionally, each circuit consumes some quantity of power, and in the aggregate these circuits can have a considerable impact on the total power consumption of an integrated circuit.
Receiver circuits can have many digital to analog converters (DACs) in any number of components, such as in preamplifiers, variable offset comparators and latched amplifiers. Each DAC occupies an amount of chip area based on the bit size of the DAC. For example, a 4-bit DAC has 24 (16) transistors and an 8-bit DAC has 28 (256) transistors, each transistor occupying some unit area.
A receiver circuit that operates in a digital mode interprets received signals as a series of logic “1s” or logic “0s.” The received signal has some amplitude, for example from 0 to 1 volt, depending on the circuit used. A half wave of the signal represents the amplitude half way between the maximum and the minimum possible amplitudes. For example, if the amplitude varies from 0 to 1 volt, then the half wave would be 0.5 volts. Voltages above the half wave are registered as having a logic “1” and voltages below the half wave are registered as having a logic “0.” Signals can become corrupted before they reach the receiver. When a signal is corrupted, its amplitude at the receiver can be different from its amplitude at a transmitter. Therefore, signals that should be registered as having a logic “1” might be registered as logic “0,” and signals that should be registered as having a logic “0” might be registered as logic “1.”
After fabrication, receiver circuits need to be tested to ensure that minimal transmission errors will occur during device operation. Such testing is referred to as margining.